Add a "How did you hear about us" field in WooCommerce

functions.php

<?php
// Filter the checkout fields
add_filter( 'woocommerce_checkout_fields', 'patricks_woocommerce_checkout_fields' );

// add a "how did you hear about us" field - $fields is passed via the filter
function patricks_woocommerce_checkout_fields( $fields ) {
    // add the field
    $fields['order']['hear_about_us'] = array(
        'type' => 'select',
        'class' => array( 'form-row-wide' ),
        'label' => 'How did you hear about us?',
        'options' => array(
            'default' => '--select--',
            'tv' => 'TV',
            'radio' => 'Radio',
            'billboards' => 'Billboards',
            'podcasts' => 'Podcasts'
        )
    );
    return $fields;
}

thanks for the code.
but this warning appears after writing the code

Warning: Invalid argument supplied for foreach() in C:\xampp\htdocs\wp2\wordpress\wp-content\plugins\woocommerce\templates\checkout\form-billing.php on line 43